Use Twoliter.override as the source of truth for publication overrides

[cbgbt]

In #372, support was added for allowing pubsys to write to an alternative repository when publishing a kit, yet keeping the kit metadata unchanged.

While discussing the change, we decided that the use of Infra.toml and environment variables as an additional source of truth was a bit confusing. We should use Twoliter.toml and Twoliter.override as the source of truth for kit publication in the future.